1. Field of the Invention
This invention relates to a control device for use in a refrigeration circuit, and more particularly, to a device for controlling the flow volume of refrigerant in a refrigeration circuit.
2. Description of the Prior Art
FIG. 1 shows a conventional refrigeration circuit for use, for example, in an automotive air conditioning system. The circuit includes compressor 1, condenser 2, receiver dryer 3, expansion device 4, and evaporator 5 serially connected. The output of evaporator 5 is also connected to the input of compressor 1.
Expansion device 4 is a thermostatic expansion valve which is utilized in the circuit to both expand and control the flow volume of the refrigerant. The operation of expansion valve 4 is dependent upon the degree of superheating of the refrigerant at the outlet side of evaporator 5. The refrigeration circuit is usually operated within a range of five to eight degrees of the temperature at superheating to minimize the loss in compressor efficiency and to simultaneously avoid damaging the valves of compressor 1 caused by intake of refrigerant in the liquid state.
If the refrigeration circuit is utilized in an automotive air conditioning system, the flow volume of the refrigerant changes greatly in accordance with the change in the rotation speed of the engine. As a result, liquid refrigerant may be returned to the suction side of compressor 1 without changing state. Also, if there is a low flow volume, hunting phenomenon may occur.
FIG. 2 shows a second conventional refrigeration circuit including compressor 1a, condenser 2a, expansion device 6, evaporator 5a, and accumulator 7 serially connected. Expansion device 6 may be a capillary. Since the outlet of evaporator 5a is connected to the inlet of accumulator 7, the degree of superheating required at the outlet side of evaporator 5a to prevent liquid refrigerant from entering compressor 1 is near zero. Therefore, the heat exchange efficiency of evaporator 5a is greatly improved. However, if there is a low or medium refrigerant flow voluem, liquid refrigerant may be accumulated in accumulator 7 and may be intaken by compressor 1a. If in the alternative there is a high refrigerant flow volume, liquid refrigerant may accumulate in condenser 2a, reducing the capacity thereof. Therefore, the liquid refrigerant accumulated in accumulator 7a will become insufficient for operation of the system. As a result, the degree of superheating of the refrigerant needed at the outlet side of evaporator 5a increases abnormally requiring accumulator 7 to be enlarged in order to maintain sufficient volume of liquid refrigerant in the circuit.